Light-Emitting Element

ABSTRACT

Provided is a light-emitting element with high external quantum efficiency and a low drive voltage. The light-emitting element includes a light-emitting layer which contains a phosphorescent compound and a material exhibiting thermally activated delayed fluorescence between a pair of electrodes, wherein a peak of a fluorescence spectrum and/or a peak of a phosphorescence spectrum of the material exhibiting thermally activated delayed fluorescence overlap(s) with a lowest-energy-side absorption band in an absorption spectrum of the phosphorescent compound, and wherein the phosphorescent compound exhibits phosphorescence in the light-emitting layer by voltage application between the pair of electrodes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/188,493, filed Nov. 13, 2018, now allowed, which is a continuation ofU.S. application Ser. No. 15/584,830, filed May 2, 2017, now U.S. Pat.No. 10,326,093, which is a continuation of U.S. application Ser. No.15/293,744, filed Oct. 14, 2016, now U.S. Pat. No. 9,673,404, which is acontinuation of U.S. application Ser. No. 14/657,384, filed Mar. 13,2015, now U.S. Pat. No. 9,478,764, which is a continuation of U.S.application Ser. No. 13/760,301, filed Feb. 6, 2013, now U.S. Pat. No.8,981,355, which claims the benefit of a foreign priority applicationfiled in Japan as Serial No. 2012-025834 on Feb. 9, 2012, all of whichare incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to light-emitting elements usmg an organicelectroluminescence (EL) phenomenon (hereinafter such light-emittingelements are also referred to as organic EL elements).

DESCRIPTION OF THE RELATED ART

An organic EL element has been actively researched and developed. In afundamental structure of the organic EL element, a light-emitting layercontaining a light-emitting material is interposed between a pair ofelectrodes. The organic EL element has attracted attention as anext-generation flat panel display element owing to characteristics suchas feasibility of being thinner and lighter, high speed response toinput signals, and capability of direct current low voltage driving. Inaddition, a display using such a light-emitting element has a featurethat it is excellent in contrast and image quality, and has a wideviewing angle. Further, being a plane light source, the organic ELelement has been attempted to be applied as a light source such as abacklight of a liquid crystal display and a lighting device.

The emission mechanism of the organic EL element is of acarrier-injection type. That is, by application of voltage with alight-emitting layer interposed between electrodes, electrons and holesinjected from the electrodes are recombined to make a light-emittingmaterial excited, and light is emitted when the excited state relaxes tothe ground state. There can be two types of the excited states: asinglet excited state (S*) and a triplet excited state (T*). Thestatistical generation ratio of the excited states in a light-emittingelement is considered to be S*:T*=1:3.

In general, the ground state of a light-emitting organic compound is asinglet state. Therefore, light emission from the singlet excited state(S*) is referred to as fluorescence because it is caused by electrontransition between the same spin multiplicities. On the other hand,light emission from the triplet excited state (T*) is referred to asphosphorescence where electron transition occurs between different spin.multiplicities. Here, in a compound exhibiting fluorescence (hereinafterreferred to as fluorescent compound), in general, phosphorescence is notobserved at room temperature, and only fluorescence is observed.Accordingly, the internal quantum efficiency (the ratio of generatedphotons to injected carriers) in a light-emitting element containing afluorescent compound is assumed to have a theoretical limit of 25% basedon S*:T*=1:3.

On the other hand, when a compound exhibiting phosphorescence(hereinafter referred to as a phosphorescent compound) is used, theinternal quantum efficiency can be theoretically increased to 100%. Thatis, higher emission efficiency can be obtained than using a fluorescentcompound. For these reasons, a light-emitting element containing aphosphorescent compound has been actively developed in recent years inorder to achieve a high-efficiency light-emitting element. As thephosphorescent compound, an organometallic complex that has iridium orthe like as a central metal has particularly attracted attention becausesuch an organometallic complex has a high phosphorescence quantum yield.For example, an organometallic complex that has iridium as a centralmetal is disclosed as a phosphorescent material in Patent Document 1.

When a light-emitting layer of a light-emitting element is formed usingthe above-described phosphorescent compound, in order to suppressconcentration quenching or quenching due to triplet-triplet annihilationof the phosphorescent compound, the light-emitting layer is often formedsuch that the phosphorescent compound is dispersed in a matrix ofanother compound. Here, the compound serving as the matrix is called ahost material, and the compound dispersed in the matrix, such as aphosphorescent compound, is called a guest material.

REFERENCE Patent Document

[Patent Document 1] International Publication WO 00/70655 pamphlet

SUMMARY OF THE INVENTION

However, it is generally said that the light extraction efficiency of anorganic EL element is approximately 20% to 30%. Accordingly, consideringlight absorption. by a reflective electrode and a transparent electrode,the external quantum efficiency of a light-emitting element containing aphosphorescent compound has a limit of approximately 25% at most.

Further, as described above, application of organic EL elements todisplays and lightings has been considered. One of objects here is areduction in power consumption. In order to reduce power consumption, itis required to reduce the drive voltage of the organic EL element.

An object of one embodiment of the present invention is to provide alight-emitting element with high external quantum efficiency. Further,an object of one embodiment of the present invention is to provide alight-emitting element with a low drive voltage.

Note that the invention to be disclosed below aims to achieve at leastone of the above-described objects.

One embodiment of the present invention is a light-emitting elementincluding a light-emitting layer which contains a phosphorescentcompound and a thermally activated delayed fluorescence material, whichis a material exhibiting thermally activated delayed fluorescence(TADF), between a pair of electrodes, wherein a peak of a fluorescencespectrum of the material exhibiting thermally activated delayedfluorescence overlaps with a lowest-energy-side absorption band in anabsorption spectrum of the phosphorescent compound, and wherein thephosphorescent compound exhibits phosphorescence in the light-emittinglayer by voltage application between the pair of electrodes.

Note that in the present specification and the like, a fluorescencespectrum of a material exhibiting thermally activated delayedfluorescence includes a delayed fluorescence spectrum (thermallyactivated delayed fluorescence spectrum).

Here, the term “delayed fluorescence” refers to light emission havingthe same spectrum as normal fluorescence and an extremely long lifetime.The lifetime is 10⁻⁶ seconds or longer, preferably 10⁻³ seconds orlonger.

In the above light-emitting element, the difference between the energyvalue of the peak of the fluorescence spectrum of the materialexhibiting thermally activated delayed fluorescence and the energy valueof a peak of the lowest-energy-side absorption band in the absorptionspectrum of the phosphorescent compound is preferably 0.3 eV or less.

Further, one embodiment of the present invention is a light-emittingelement including a light-emitting layer which contains a phosphorescentcompound and a material exhibiting thermally activated delayedfluorescence between a pair of electrodes, wherein a peak of aphosphorescence spectrum of the material exhibiting thermally activateddelayed fluorescence overlaps with a lowest-energy-side absorption bandin an absorption spectrum of the phosphorescent compound, and whereinthe phosphorescent compound exhibits phosphorescence in thelight-emitting layer by voltage application between the pair ofelectrodes.

In the above light-emitting element, the difference between the energyvalue of the peak of the phosphorescence spectrum of the materialexhibiting thermally activated delayed fluorescence and the energy valueof a peak of the lowest-energy-side absorption band in the absorptionspectrum of the phosphorescent compound is preferably 0.3 eV or less.

Further, one embodiment of the present invention is a light-emittingelement including a light-emitting layer which contains a phosphorescentcompound and a material exhibiting thermally activated delayedfluorescence between a pair of electrodes, wherein a peak of afluorescence spectrum and a peak of a phosphorescence spectrum of thematerial exhibiting thermally activated delayed fluorescence eachoverlap with a lowest-energy-side absorption band in an absorptionspectrum of the phosphorescent compound, and wherein the phosphorescentcompound exhibits phosphorescence in the light-emitting layer by voltageapplication between the pair of electrodes.

In the above light-emitting element, the difference, between the energyvalue of the peak of the fluorescence spectrum of the materialexhibiting thermally activated delayed fluorescence and the energy valueof a peak of the lowest-energy-side absorption band in the absorptionspectrum of the phosphorescent compound, and the difference between theenergy value of the peak of the phosphorescence spectrum of the materialexhibiting thermally activated delayed fluorescence and the energy valueof the peak of the lowest-energy-side absorption band in the absorptionspectrum of the phosphorescent compound, are each preferably 0.3 eV orless.

In the above light-emitting element, the difference between the energyvalue of the peak of the fluorescence spectrum and the energy value ofthe peak of the phosphorescence spectrum of the material exhibitingthermally activated delayed fluorescence is preferably 0.3 eV or less.

In any of the above light-emitting elements, the lowest-energy-sideabsorption band in the absorption spectrum of the phosphorescentcompound preferably includes an absorption band based on a tripletMLCT(metal to ligand charge transfer) transition.

In any of the above light-emitting elements, the phosphorescent compoundis preferably an organometallic complex, more preferably an iridiumcomplex.

In any of the above light-emitting elements, the molar absorptioncoefficient of the lowest-energy-side absorption band in the absorptionspectrum of the phosphorescent compound is preferably higher than orequal to 5000 /M·cm.

In any of the above light-emitting elements, the material exhibitingthermally activated delayed fluorescence is preferably a heterocycliccompound including a π-electron excess heteroaromatic ring and aπ-electron deficient heteroaromatic ring, more preferably a heterocycliccompound in which the π-electron excess heteroaromatic ring is directlybonded to the π-electron deficient heteroaromatic ring.

The light-emitting element of one embodiment of the present inventioncan be applied to a light-emitting device, an electronic device, and alighting device.

According to one embodiment of the present invention, a light-emittingelement with high external quantum efficiency can be provided. Accordingto another embodiment of the present invention, a light-emitting elementwith a low drive voltage can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 shows a concept of one embodiment of the present invention; and

FIGS. 2A to 2F illustrate light-emitting elements of embodiments of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described in detail withreference to the accompanying drawings. Note that the invention is notlimited to the following description, and it will be easily understoodby those skilled in the art that various changes and modifications canbe made without departing from the spirit and scope of the invention.Therefore, the invention should not be construed as being limited to thedescription in the following embodiments. Note that in the structures ofthe invention described below, the same portions or portions havingsimilar functions are denoted by the same reference numerals indifferent drawings, and description of such portions is not repeated.

Embodiment 1

In this embodiment, a light-emitting element of one embodiment of thepresent invention will be described with reference to FIG. 1.

The light-emitting element of one embodiment of the present inventionincludes a light-emitting layer containing a guest material, i.e., alight-emitting material, and a host material in which the guest materialis dispersed. As the guest material, a phosphorescent compound is used.As the host material, a material exhibiting thermally activated delayedfluorescence is used. FIG. 1 shows a conceptual schematic view of aphosphorescence spectrum and a fluorescence spectrum of the hostmaterial in the light-emitting element of one embodiment of the presentinvention and an absorption spectrum of the phosphorescent compound. InFIG. 1, the vertical axes represent absorption intensity and emissionintensity, and the horizontal axis represents energy.

In the light-emitting layer in the light-emitting element of oneembodiment of the present invention, the content of the host material ishigher than that of the guest material. The structure in which the guestmaterial is dispersed in the host material can prevent thelight-emitting layer from crystallizing. Further, it is possible tosuppress concentration quenching due to high concentration of the guestmaterial, and thus the light-emitting element can have higher emissionefficiency.

According to this embodiment, it is preferable that the level of atriplet excitation energy (T₁ level) of the host material be higher thanthat of the guest material. This is because, when the T₁ level of thehost material is lower than that of the guest material, the tripletexcitation energy of the guest material, which is to contribute to lightemission, is quenched by the host material and accordingly the emissionefficiency is decreased.

Elementary Processes of Light Emission

First, a description is given of general elementary processes of lightemission in a light-emitting element using a phosphorescent compound asa guest material.

(1) The case where an electron and a hole are recombined in a guestmolecule, and the guest molecule is excited (direct recombinationprocess).

(1-1) When the excited state of the guest molecule is a triplet excitedstate, the guest molecule emits phosphorescence.

(1-2) When the excited state of the guest molecule is a singlet excitedstate, the guest molecule in the singlet excited state undergoesintersystem crossing to a triplet excited state and emitsphosphorescence.

In other words, in the direct recombination process in (1), as long asthe efficiency of intersystem crossing and the phosphorescence quantumyield of the guest molecule are high, high emission efficiency can beobtained. As described above, it is preferable that the T₁ level of thehost molecule be higher than that of the guest molecule.

(2) The case where an electron and a hole are recombined in a hostmolecule and the host molecule is excited (energy transfer process).

(2-1) When the excited state of the host molecule is a triplet excitedstate and the T₁ level of the host molecule is higher than that of theguest molecule, an excitation energy transfers from the host molecule tothe guest molecule, and thus, the guest molecule is put in a tripletexcited state. The guest molecule in the triplet excited state exhibitsphosphorescence. Note that the energy can transfer to a singletexcitation energy level (S₁ level) of the guest molecule in theory;however, since the S₁ level of the guest molecule has a higher energythan the T₁ level of the host molecule in many cases, energy transfer tothe S₁ level of the guest molecule is barely likely to be a main energytransfer process; therefore, a description thereof is not given here.

(2-2) When the excited state of the host molecule is a singlet excitedstate and the S₁ level of the host molecule is higher than the S₁ leveland T₁ level of the guest molecule, an excitation energy transfers fromthe host molecule to the guest molecule, and thus, the guest molecule isput in a singlet excited state or a triplet excited state. The guestmolecule in the triplet excited state exhibits phosphorescence. Inaddition, the guest molecule in the singlet excited state undergoesintersystem crossing to a triplet excited state and emitsphosphorescence.

In other words, in the energy transfer process in (2), it is importanthow efficiently both the triplet excitation energy and the singletexcitation energy of the host molecule can transfer to the guestmolecule.

Energy Transfer Process

The following will show energy transfer processes between molecules indetail.

First, as a mechanism of energy transfer between molecules, thefollowing two mechanisms are proposed. A molecule donating excitationenergy is referred to as a host molecule, while a molecule accepting theexcitation energy is referred to as a guest molecule.

Förster Mechanism (Dipole-Dipole Interaction)

In Förster mechanism (also referred to as Förster resonance energytransfer), direct intermolecular contact is not necessary for energytransfer. Through a resonant phenomenon of dipolar oscillation between ahost molecule and a guest molecule, energy transfer occurs. The resonantphenomenon of dipolar oscillation causes the host molecule to donateenergy to the guest molecule; thus, the host molecule relaxes to aground state and the guest molecule is excited. The rate constantk_(h*→g) of Förster mechanism is expressed by a formula (1).

$\begin{matrix}{\left\lbrack {{FORMULA}\mspace{14mu} 1} \right\rbrack \mspace{585mu}} & \; \\{k_{h^{*}\rightarrow g} = {\frac{9000\mspace{11mu} c^{4}\kappa^{2}\varphi \; \ln \; 10}{128\; \pi^{2}n^{4}N\; \tau \; R^{6}}{\int{\frac{{f_{h}^{\prime}(v)}{ɛ_{g}(v)}}{v^{4}}{dv}}}}} & (1)\end{matrix}$

In the formula (1), ν denotes a frequency, ƒ′_(h)(ν) denotes anormalized emission spectrum of a host molecule (a fluorescence spectrumin energy transfer from a singlet excited state, and a phosphorescencespectrum in energy transfer from a triplet excited state), ε_(g)(ν)denotes a molar absorption coefficient of a guest molecule, N denotesAvogadro's number, n denotes a refractive index of a medium, R denotesan intermolecular distance between the host molecule and the guestmolecule, τ denotes a measured lifetime of an excited state(fluorescence lifetime or phosphorescence lifetime), c denotes the speedof light, ϕ denotes a luminescence quantum yield (a fluorescence quantumyield in energy transfer from a singlet excited state, and aphosphorescence quantum yield in energy transfer from a triplet excitedstate), and κ² denotes a coefficient (0 to 4) of orientation of atransition dipole moment between the host molecule and the guestmolecule. Note that κ²=⅔ in random orientation.

(Dexter Mechanism (Electron Exchange Interaction)

In Dexter mechanism (also referred to as Dexter electron transfer), ahost molecule and a guest molecule are close to a contact effectiverange where their orbitals overlap, and the host molecule in an excitedstate and the guest molecule in a ground state exchange their electrons,which leads to energy transfer. The rate constant k_(h*→g) of Dextermechanism is expressed by a formula (2).

$\begin{matrix}{\left\lbrack {{FORMULA}\mspace{20mu} 2} \right\rbrack \mspace{580mu}} & \; \\{k_{h^{*}\rightarrow g} = {\left( \frac{2\pi}{h} \right)K^{2}{\exp \left( {- \frac{2R}{L}} \right)}{\int{{f_{h}^{\prime}(v)}{ɛ_{g}^{\prime}(v)}{dv}}}}} & (2)\end{matrix}$

In the formula (2), h denotes a Planck constant, K denotes a constanthaving an energy dimension, ν denotes a frequency, ƒ′_(h)(ν) denotes anormalized emission spectrum of a host molecule (a fluorescence spectrumin energy transfer from a singlet excited state, and a phosphorescencespectrum in energy transfer from a triplet excited state), ε′_(g)(ν)denotes a normalized absorption spectrum of a guest molecule, L denotesan effective molecular radius, and R denotes an intermolecular distancebetween the host molecule and the guest molecule.

Here, the efficiency of energy transfer from the host molecule to theguest molecule (energy transfer efficiency Φ_(ET)) is thought to beexpressed by a formula (3). In the formula, k_(r) denotes a rateconstant of a light-emission process (fluorescence in energy transferfrom a singlet excited state, and phosphorescence in energy transferfrom a triplet excited state) of the host molecule, k_(n) denotes a rateconstant of a non-light-emission process (thermal deactivation orintersystem crossing) of the host molecule, and τ denotes a measuredlifetime of the excited state of the host molecule.

$\begin{matrix}{\left\lbrack {{FORMULA}\mspace{20mu} 3} \right\rbrack \mspace{580mu}} & \; \\{\Phi_{ET} = {\frac{k_{h^{*}\rightarrow g}}{k_{r} + k_{n} + k_{h^{*}\rightarrow g}} = \frac{k_{h^{*}\rightarrow g}}{\left( \frac{1}{\tau} \right) + k_{h^{*}\rightarrow g}}}} & (3)\end{matrix}$

First, according to the formula (3), in order to increase the energytransfer efficiency Φ_(ET), the rate constant k_(h*→g) of energytransfer may be further increased as compared with another rate constantk_(r)+k_(n)(=1/τ). Then, in order to increase the rate constant k_(h*→g)of energy transfer, based on the formulas (1) and (2), in both Förstermechanism and Dexter mechanism, it is preferable that an emissionspectrum of a host molecule (a fluorescence spectrum in energy transferfrom a singlet excited state, and phosphorescence spectrum in energytransfer from a triplet excited state) largely overlap with anabsorption spectrum of a guest molecule.

Here, the present inventor has considered that a lowest-energy-side(longest-wavelength-side) absorption band in the absorption spectrum ofthe guest molecule is important in considering the overlap between theemission spectrum of the host molecule and the absorption spectrum ofthe guest molecule.

In this embodiment, a phosphorescent compound is used as the guestmaterial. In an absorption spectrum of the phosphorescent compound, anabsorption band that is considered to contribute to light emission mostgreatly includes an absorption wavelength corresponding to directtransition from a singlet ground state to a triplet excitation state andthe vicinity of the absorption wavelength. The absorption band is on thelongest wavelength side (lowest energy side). Therefore, it isconsidered preferable that the emission spectrum (a fluorescencespectrum and a phosphorescence spectrum) of the host material overlapwith the lowest-energy-side absorption band in the absorption spectrumof the phosphorescent compound.

For example, most organometallic complexes, especially light-emittingiridium complexes, have a broad absorption band around 2.0 eV to 2.5 eVas the lowest-energy-side absorption band (as a matter of fact, thebroad absorption band can be on a lower or higher energy side). Thisabsorption band is mainly based on a triplet MLCT transition. Note thatit is considered that the absorption band also includes absorption bandsbased on a triplet π-π* transition and a singlet MLCT transition, andthat these absorption bands overlap with each other to form a broadabsorption band on the lowest energy side in the absorption spectrum. Inother words, it is considered that the difference between the lowestsinglet excited state and the lowest triplet excited state is small, andabsorption bands based on these states overlap with each other to form abroad absorption band on the lowest energy side in the absorptionspectrum. Therefore, as described above, when an organometallic complex(especially iridium complex) is used as the guest material, it ispreferable that the lowest-energy-side broad absorption band largelyoverlap with the emission spectrum of the host material.

From the above discussion, it is preferable that, in energy transferfrom the host material in a triplet excited state, the phosphorescencespectrum of the host material and the lowest-energy-side absorption bandof the guest material largely overlap with each other. It is alsopreferable that, in energy transfer from the host material in a singletexcited state, the fluorescence spectrum of the host material and thelowest-energy-side absorption band of the guest material largely overlapwith each other.

That is, in order to efficiently perform energy transfer from the hostmaterial regardless of being in the triplet excited state or the singletexcited state, it is clear from the above discussion that the hostmaterial needs to be designed so as to have both the phosphorescencespectrum and fluorescence spectrum overlapping with thelowest-energy-side absorption band of the guest material.

However, the S₁ level generally differs greatly from the T₁ level (S₁level>T₁ level); therefore, the fluorescence emission wavelength alsodiffers greatly from the phosphorescence emission wavelength(fluorescence emission wavelength<phosphorescence emission wavelength).For example, 4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP), which iscommonly used as a host material in a light-emitting element containinga phosphorescent compound, has a phosphorescence spectrum around 500 nm(about 3.1 eV) and a fluorescence spectrum around 400 nm (about 2.5 eV),which are largely different by about 100 nm (the difference is 0.6 eV ormore in energy). This example also shows that it is extremely difficultto design a host material so as to have its fluorescence spectrum in aposition similar to that of its phosphorescence spectrum.

Here, the light-emitting element of one embodiment of the presentinvention contains a material exhibiting thermally activated delayedfluorescence as a host material.

One embodiment of the present invention is a light-emitting elementincluding a light-emitting layer which contains a phosphorescentcompound and a material exhibiting thermally activated delayedfluorescence between a pair of electrodes, wherein a peak of afluorescence spectrum of the material exhibiting thermally activateddelayed fluorescence overlaps with a lowest-energy-side absorption bandin an absorption spectrum of the phosphorescent compound, and whereinthe phosphorescent compound exhibits phosphorescence in thelight-emitting layer by voltage application between the pair ofelectrodes.

Another embodiment of the present invention is a light-emitting elementincluding a light-emitting layer which contains a phosphorescentcompound and a material exhibiting thermally activated delayedfluorescence between a pair of electrodes, wherein a peak of aphosphorescence spectrum of the material exhibiting thermally activateddelayed fluorescence overlaps with a lowest-energy-side absorption bandin an absorption spectrum of the phosphorescent compound, and whereinthe phosphorescent compound exhibits phosphorescence in thelight-emitting layer by voltage application between the pair ofelectrodes.

The material exhibiting thermally activated delayed fluorescence has asmall difference between the lowest triplet excitation energy and thelowest singlet excitation energy. In other words, the emission spectrumof the material exhibiting thermally activated delayed fluorescence fromthe singlet state and the emission spectrum thereof from the tripletstate are close to each other. Therefore, when a peak of a fluorescenceor phosphorescence spectrum of the material exhibiting thermallyactivated delayed fluorescence is designed so as to overlap with thelowest-energy-side absorption band of the phosphorescent compound, boththe fluorescence and phosphorescence spectra of the material exhibitingthermally activated delayed fluorescence overlap with (or become veryclose to) the lowest-energy-side absorption band of the phosphorescentcompound (see FIG. 1). This means that energy can efficiently transferto the phosphorescent compound from the material exhibiting thei wallyactivated delayed fluorescence regardless of being in the singlet stateor in the triplet state.

Since the peak of the fluorescence or phosphorescence spectrum of thematerial exhibiting thermally activated delayed fluorescence overlapswith the lowest-energy-side absorption band in the absorption spectrumof the phosphorescent compound, energy smoothly transfers from thematerial exhibiting thermally activated delayed fluorescence, regardlessof being in the singlet excited state or in the triplet excited state,to the phosphorescent compound. As a result, the light-emitting elementof one embodiment of the present invention has high energy transferefficiency. Thus, according to one embodiment of the present invention,a light-emitting element with high external quantum efficiency can beachieved.

In view of the above-described energy transfer processes, before theexcitation energy of the host molecule transfers to the guest molecule,when the host molecule itself is deactivated by emitting the excitationenergy as light or heat, the emission efficiency is decreased and thelifetime is shortened. According to one embodiment of the presentinvention, however, the energy smoothly transfers, so that thedeactivation of the excitation energy can be suppressed. Thus, alight-emitting element with a long lifetime can be achieved.

Here, it is preferable that the peak of the fluorescence spectrum of thematerial exhibiting thermally activated delayed fluorescence be not toohigh (the wavelength thereof be not too short) because in that case ahigher voltage is required to transfer energy from the materialexhibiting thermally activated delayed fluorescence to thephosphorescent compound so that the phosphorescent compound can emitlight, resulting in extra energy consumption.

In this light, in one embodiment of the present invention, the energy ofthe peak of the fluorescence spectrum of the material exhibitingthermally activated delayed fluorescence is preferably as low aspossible (the wavelength thereof is preferably as long as possible)because in that case the emission starting voltage of the light-emittingelement is low. Since the energy of the peak of the fluorescencespectrum of the material exhibiting thermally activated delayedfluorescence is low, the light-emitting element of one embodiment of thepresent invention is driven at a low voltage and has high emissionefficiency (high external quantum efficiency), and thus has high powerefficiency.

Further, in this light, within a range where the peak of thefluorescence or phosphorescence spectrum of the material exhibitingthermally activated delayed fluorescence overlaps with thelowest-energy-side absorption band in the absorption spectrum of thephosphorescent compound, the energy of the peak of the fluorescencespectrum may be lower than that of a peak of the absorption band. Thisis because in this case, the light-emitting element can be driven at alow voltage with relatively high energy efficiency kept.

In particular, both the peaks of the fluorescence and phosphorescencespectra of the material exhibiting thermally activated delayedfluorescence preferably overlap with the lowest-energy-side absorptionband in the absorption spectrum of the phosphorescent compound, becausein that case a light-emitting element with particularly high energytransfer efficiency and particularly high external quantum efficiencycan be achieved.

Specifically, another embodiment of the present invention is alight-emitting element including a light-emitting layer which contains aphosphorescent compound and a material exhibiting thermally activateddelayed fluorescence between a pair of electrodes, wherein a peak of afluorescence spectrum and a peak of a phosphorescence spectrum of thematerial exhibiting thermally activated delayed fluorescence eachoverlap with a lowest-energy-side absorption band in an absorptionspectrum of the phosphorescent compound, and wherein the phosphorescentcompound exhibits phosphorescence in the light-emitting layer by voltageapplication between the pair of electrodes.

To make the emission spectrum of the material exhibiting thermallyactivated delayed fluorescence overlap sufficiently with the absorptionspectrum of the phosphorescent compound, the difference between theenergy value of the peak of the fluorescence spectrum of the materialexhibiting thermally activated delayed fluorescence and the energy valueof the peak of the lowest-energy-side absorption band in the absorptionspectrum of the phosphorescent compound is preferably 0.3 eV or less,more preferably 0.2 eV or less, even more preferably 0.1 eV or less.Further, the difference between the energy value of the peak of thephosphorescence spectrum of the material exhibiting thermally activateddelayed fluorescence and the energy value of the peak of thelowest-energy-side absorption band in the absorption spectrum of thephosphorescent compound is preferably 0.3 eV or less, more preferably0.2 eV or less, even more preferably 0.1 eV or less.

As described above, the lowest triplet excitation energy' and the lowestsinglet excitation energy of the material exhibiting thermally activateddelayed fluorescence, which is used as the host material in thelight-emitting element of one embodiment of the present invention, areclose to each other. In particular, in the light-emitting element of oneembodiment of the present invention, the difference between the energyvalues of the peaks of the fluorescence and phosphorescence spectra ofthe material exhibiting thermally activated delayed fluorescence ispreferably 0.3 eV or less.

In the above light-emitting element, it is preferable that thelowest-energy-side absorption band in the absorption spectrum of thephosphorescent compound include an absorption band based on a tripletMLCT transition. A triplet MLCT excited state is the lowest tripletexcited state of the phosphorescent compound which is the guestmaterial, and thus, the phosphorescent compound exhibits phosphorescencetherefrom. That is, phosphorescence from the triplet MIXT excited stateinvolves few deactivation processes other than light emission, and thus,it is considered that high emission efficiency can be obtained by makingthe proportion of this excited state as high as possible. For thesereasons, there are preferably many energy transfer processes whereenergy directly transfers from the material exhibiting thermallyactivated delayed fluorescence to the triplet MLCT excited state byusing the absorption band based on the triplet MLCT transition. In theabove light-emitting element, the phosphorescent compound is preferablyan organometallic complex, more preferably an iridium complex.

Here, it is preferable that sufficient excitation energy of the materialexhibiting thermally activated delayed fluorescence transfer to thephosphorescent compound, and that fluorescence (delayed fluorescence)from the singlet excited state be not substantially observed.

Further, in the energy transfer from the material exhibiting thermallyactivated delayed fluorescence in the singlet excited state, the Förstermechanism is considered to be important. Considering that, from theformula (1), the molar absorption coefficient of the lowest-energy-sideabsorption band of the phosphorescent compound is preferably 2000 /M·cmor higher, more preferably 5000 M·cm or higher.

Note that this embodiment can be combined with the other embodiment asappropriate.

Embodiment 2

In Embodiment 2, light-emitting elements each according to oneembodiment of the present invention will be described with reference toFIGS. 2A to 2F.

Each of the light-emitting elements shown in this embodiment includes apair of electrodes (a first electrode and a second electrode) and an ELlayer(s) provided between the pair of electrodes. One of the electrodesserves as an anode and the other serves as a cathode. The EL layer(s)includes at least a light-emitting layer, and the light-emitting layercontains a guest material, i.e., a light-emitting material, and a hostmaterial in which the guest material is dispersed. As the guestmaterial, a phosphorescent compound is used. As the host material, amaterial exhibiting thermally activated delayed fluorescence is used.

The light-emitting element of one embodiment of the present inventioncan employ any of a top emission structure, a bottom emission structure,and a dual emission structure.

The following will show specific examples of a structure of thelight-emitting element of one embodiment of the present invention.

A light-emitting element illustrated in FIG. 2A includes an EL layer 203between a first electrode 201 and a second electrode 205. In thisembodiment, the first electrode 201 serves as the anode, and the secondelectrode 205 serves as the cathode.

When a voltage higher than the threshold voltage of the light-emittingelement is applied between the first electrode 201 and the secondelectrode 205, holes are injected to the EL layer 203 from the firstelectrode 201 side and electrons are injected to the EL layer 203 fromthe second electrode 205 side. The injected electrons and holes arerecombined in the EL layer 203 and a light-emitting material containedin the EL layer 203 emits light.

The EL layer 203 includes at least a light-emitting layer, as describedabove. In addition to the light-emitting layer, the EL layer 203 mayfurther include one or more layers containing any of a material with ahigh hole-injection property, a material with a high hole-transportproperty, a hole-blocking material, a material with a highelectron-transport property, a material with a high electron-injectionproperty, a bipolar property (a material with a high electron- andhole-transport property), and the like.

A known material can be used for the EL layer 203. Either a lowmolecular compound or a high molecular compound can be used, and aninorganic compound may also be used.

A specific example of a structure of the EL layer 203 is illustrated inFIG. 2B. In the EL layer 203 illustrated in FIG. 2B, a hole-injectionlayer 301, a hole-transport layer 302, a light-emitting layer 303, anelectron-transport layer 304, and an electron-injection layer 305 arestacked in this order from the first electrode 201 side.

A light-emitting element illustrated in FIG. 2C includes the EL layer203 between the first electrode 201 and the second electrode 205, andfurther includes an intermediate layer 207 between the EL layer 203 andthe second electrode 205.

A specific example of a structure the intermediate layer 207 isillustrated in FIG. 2D. The intermediate layer 207 includes at least acharge-generation region 308. In addition to the charge-generationregion 308, the intermediate layer 207 may further include anelectron-relay layer 307 and an electron-injection buffer layer 306.

When a voltage higher than the threshold voltage of the light-emittingelement is applied between the first electrode 201 and the secondelectrode 205, holes and electrons are generated in thecharge-generation region 308, and the holes move into the secondelectrode 205 and the electrons move into the electron-relay layer 307.The electron-relay layer 307 has a high electron-transport property andimmediately transfers the electrons generated in the charge-generationregion 308 to the electron-injection buffer layer 306. Theelectron-injection buffer layer 306 reduces a barrier to electroninjection into the EL layer 203, so that the efficiency of the electroninjection into the EL layer 203 is increased. Thus, the electronsgenerated in the charge-generation region 308 are injected into the LUMOlevel of the EL layer 203 through the electron-relay layer 307 and theelectron-injection buffer layer 306.

In addition, the electron-relay layer 307 can prevent reaction at theinterface between a material contained in the charge-generation region308 and a material contained in the electron-injection buffer layer 306.Thus, it is possible to prevent interaction such as damaging thefunctions of the charge-generation region 308 and the electron-injectionbuffer layer 306.

As illustrated in light-emitting elements in FIGS. 2E and 2F, aplurality of EL layers may be stacked between the first electrode 201and the second electrode 205. In this case, the intermediate layer 207is preferably provided between the stacked EL layers. For example, thelight-emitting element illustrated in FIG. 2E includes the intermediatelayer 207 between a first EL layer 203 a and a second EL layer 203 b.The light-emitting element illustrated in FIG. 2F includes n EL layers(n is a natural number of 2 or more) and the intermediate layers 207, anintermediate layer 207 being between an m-th EL layer 203(m) On is anatural number of 1 to (n−1)) and an m+1)-th EL layer 203(m+1).

The following will show behaviors of electrons and holes in theintermediate layer 207 between the EL layer 203(m) and the EL layer203(m+1). When a voltage higher than the threshold voltage of thelight-emitting element is applied between the first electrode 201 andthe second electrode 205, holes and electrons are generated in theintermediate layer 207, and the holes move into the EL layer 203(m+1)provided on the second electrode 205 side and the electrons move intothe EL layer 203(m) provided on the first electrode 201 side. The holesinjected into the EL layer 203(m+1) are recombined with the electronsinjected from the second electrode 205 side, so that a light-emittingmaterial contained in the EL layer 203(m+1) emits light. Further, theelectrons injected into the EL layer 203(m) are recombined with theholes injected from the first electrode 201 side, so that alight-emitting material contained in the EL layer 203(m) emits light.Thus, the holes and electrons generated in the intermediate layer 207cause light emission in the respective EL layers.

Note that the EL layers can be provided in contact with each other aslong as the same structure as the intermediate layer is formedtherebetween. For example, when the charge-generation region is formedover one surface of an EL layer, another EL layer can be provided incontact with the surface.

Further, by forming EL layers to emit light of different colors fromeach other, a light-emitting element as a whole can provide lightemission of a desired color. For example, by forming a light-emittingelement having two EL layers such that the emission color of the firstEL layer and the emission color of the second EL layer are complementarycolors, the light-emitting element can provide white light emission as awhole. Note that the word “complementary” means color relationship inwhich an achromatic color is obtained when colors are mixed. That is,white light emission can be obtained by mixture of light from materialswhose emission colors are complementary colors. This can be applied to alight-emitting element having three or more EL layers.

FIGS. 2A to 2F can be used in an appropriate combination. For example,the intermediate layer 207 can be provided between the second electrode205 and the EL layer 203(n) in FIG. 2F.

Examples of materials which can be used for each layer will be describedbelow. Note that each layer is not limited to a single layer, but may bea stack of two or more layers.

Anode

The electrode serving as the anode (the first electrode 201 in thisembodiment) can be formed using one or more kinds of conductive metals,alloys, conductive compounds, and the like. In particular, it ispreferable to use a material with a high work function (4.0 eV or more).Examples include indium tin oxide (ITO), indium tin oxide containingsilicon or silicon oxide, indium zinc oxide, indium oxide containingtungsten oxide and zinc oxide, graphene, gold, platinum, nickel,tungsten, chromium, molybdenum, iron, cobalt, copper, palladium, and anitride of a metal material (e.g., titanium nitride).

When the anode is in contact with the charge-generation region, any of avariety of conductive materials can be used regardless of their workfunctions; for example, aluminum, silver, an alloy containing aluminum,or the like can be used.

Cathode

The electrode serving as the cathode (the second electrode 205 in thisembodiment) can be formed using one or more kinds of conductive metals,alloys, conductive compounds, and the like. In particular, it ispreferable to use a material with a low work function (3.8 eV or less).Examples include aluminum, silver, an element belonging to Group 1 or 2of the periodic table (e.g., an alkali metal such as lithium or cesium,an alkaline earth metal such as calcium or strontium, or magnesium), analloy containing any of these elements (e.g., Mg—Ag or Al—Li), a rareearth metal such as europium or ytterbium, and an alloy containing anyof these rare earth metals.

When the cathode is in contact with the charge-generation region, any ofa variety of conductive materials can be used regardless of their workfunctions; for example, ITO, indium tin oxide containing silicon orsilicon oxide, or the like can be used.

The light-emitting element may have a structure in which one of theanode and the cathode is formed using a conductive film that transmitsvisible light and the other is formed using a conductive film thatreflects visible light, or a structure in which both the anode and thecathode are formed using conductive films that transmit visible light.

The conductive film that transmits visible light can be formed using,for example, indium oxide, ITO, indium zinc oxide, zinc oxide, or zincoxide to which gallium is added. Alternatively, a film of a metalmaterial such as gold, platinum, nickel, tungsten, chromium, molybdenum,iron, cobalt, copper, palladium, or titanium, or a nitride of any ofthese metal materials (e.g., titanium nitride) can be formed thin so asto have a light-transmitting property. Further alternatively, grapheneor the like may be used.

The conductive film that reflects visible light can be formed using, forexample, a metal material such as aluminum, gold, platinum, silver,nickel, tungsten, chromium, molybdenum, iron, cobalt, copper, orpalladium; an aluminum-containing alloy (aluminum alloy) such as analloy of aluminum and titanium, an alloy of aluminum and nickel, or analloy of aluminum and neodymium; or a silver-containing alloy such as analloy of silver and copper. An alloy of silver and copper is preferablebecause of its high heat resistance. Further, lanthanum, neodymium, orgermanium may be added to the metal material or the alloy.

The electrodes may be formed separately by a vacuum evaporation methodor a sputtering method. Alternatively, when a silver paste or the likeis used, a coating method or an inkjet method may be used.

Hole-injection layer 301

The hole-injection layer 301 contains a material with a highhole-injection property.

Examples of the material with a high hole-injection property includemetal oxides such as molybdenum oxide, titanium oxide, vanadium oxide,rhenium oxide, ruthenium oxide, chromium oxide, zirconium oxide, hafniumoxide, tantalum oxide, silver oxide, tungsten oxide, and manganeseoxide.

Alternatively, it is possible to use a phthalocyanine-based compoundsuch as phthalocyanine (abbreviation: H₂Pc), or copper(II)phthalocyanine (abbreviation: CuPc),

Further alternatively, it is possible to use an aromatic amine compoundwhich is a low molecular organic compound, such as4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyI)-N-phenylamino]triphenylamine(abbreviation: MTDATA),4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB),4,4′-bis(N-{4-[N′-(3-methylphenyl)-N′-phenylamino]phenyl}-N-phenylamino)biphenyl(abbreviation: DNTPD),1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B),3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2), or3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1).

Further alternatively, it is possible to use a high molecular compoundsuch as poly(N-vinylcarbazole) (abbreviation: PVK),poly(4-vinyltriphenylamine) (abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide] (abbreviation: PTPDMA), orpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation:Poly-TPD), or a high molecular compound to which acid is added, such aspoly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT/PSS)or polyaniline/poly(styrenesulfonic acid) (PAni/PSS).

The hole-injection layer 301 may serve as the charge-generation region.When the hole-injection layer 301 in contact with the anode serves asthe charge-generation region, a variety of conductive materials can beused for the anode regardless of their work functions. Materialscontained in the charge-generation region will be described later.

Hole-Transport Layer 302

The hole-transport layer 302 contains a material with a highhole-transport property.

The material with a high hole-transport property is preferably amaterial with a property of transporting more holes than electrons, andis especially preferably a material with a hole mobility of 10⁻⁶ cm²/V·sor more.

For example, it is possible to use an aromatic amine compound such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB orα-NPB),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP),4,4′-bis[N-(9,9-dimethylfluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: DFLDPBi), or4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB).

Alternatively, it is possible to use a carbazole derivative such as4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole (abbreviation: CzPA),or 9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: PCzPA).

Further alternatively, it is possible to use an aromatic hydrocarboncompound such as 2-tert-butyl-9,10-di(2-naphthyl)anthracene(abbreviation: t-BuDNA), 9,10-di(2-naphthyl)anthracene (abbreviation:DNA), or 9,10-diphenylanthracene (abbreviation: DPAnth).

Further alternatively, it is possible to use a high molecular compoundsuch as PVK, PVTPA, PTPDMA, or Poly-TPD.

Light-Emitting Layer 303

The light-emitting layer 303 contains a guest material, i.e., alight-emitting material, and a host material in which the guest materialis dispersed. As the guest material, a phosphorescent compoundexhibiting phosphorescence is used. As the host material, a materialexhibiting thermally activated delayed fluorescence is used.

As the phosphorescent compound, which is the guest material, anorganometallic complex is preferably used, and an iridium complex isparticularly preferably used. In consideration of energy transfer due toFörster mechanism described above, the molar absorption coefficient ofthe-lowest-energy-side absorption band of the phosphorescent compound ispreferably 2000/M·cm or higher, more preferably 5000/M·cm or higher. Asa compound having such a high molar absorption coefficient, aphosphorescent organometallic iridium complex having an aryl diazine(pyridazine, pyrimidine, or pyrazine to which an aryl group is bonded)as a ligand is preferably used. In particular, a phosphorescentortho-metalated iridium complex in which a carbon atom of a phenyl groupof a phenylpyrimidine derivative or a phenylpyrazine derivative isbonded to an iridium atom is preferably used. Specifically, it ispossible to usebis(3,5-dimethyl-2-phenylpyrazinato)(dipivaloylmethanato)iridium(III)(abbreviation: [Ir(mppr-Me)₂(dpm)]),(acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III) (anothername:bis[2-(6-phenyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ²O,O′)iridium(III))(abbreviation: [Ir(dppm)₂(acac)]),bis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III)(abbreviation: [Ir(tppr)₂(dpm)]),(acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium(III) (anothername:bis[2-(6-methyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ²O,O′)iridium(III)(abbreviation: [Ir(mppm)₂(acac)]),(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)(another name:bis[2-(6-tert-butyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ²O,O′)iridium(III)(abbreviation: [Ir(tBuppm)₂(acac)]), or the like.

As the host material, a known material exhibiting thermally activateddelayed fluorescence can be used. Examples of the material exhibitingthermally activated delayed fluorescence include a fullerene, aderivative thereof, an acridine derivative such as proflavine, andeosin.

As the material exhibiting thermally activated delayed fluorescence, ametal-containing porphyrin can be used, such as a porphyrin containingmagnesium (Mg), zinc (Zn), cadmium (Cd), tin (Sn), platinum (Pt), indium(In), or palladium (Pd). Examples of the metal-containing porphyrininclude a protoporphyrin-tin fluoride complex (abbreviation: SnF₂(ProtoIX)), a mesoporphyrin-tin fluoride complex (abbreviation: SnF₂(MesoIX)), a hematoporphyrin-tin fluoride complex (abbreviation: SnF₂(HematoIX)), a coproporphyrin tetramethyl ester-tin fluoride complex(abbreviation: SnF₂(Copro III-4Me)), an octaethylporphyrin-tin fluoridecomplex (abbreviation: SnF₂(OEP)), an etioporphyrin-tin fluoride complex(abbreviation: SnF₂(Etio I)), and an octaethylporphyrin-platinumchloride complex (abbreviation: PtCl₂(OEP)), which are shown in thefollowing structural formulas.

Alternatively, as the material exhibiting thermally activated delayedfluorescence, a heterocyclic compound including a π-electron excessheteroaromatic ring and a π-electron deficient heteroaromatic ring canbe used, such as2-(biphenyl-4-yl)-4,6-bis(12-phenylindolo[2,3-α]carbazol-11-yl)-1,3,5-triazine(abbreviation: PIC-TRZ), which is shown in the following structuralformula. The heterocyclic compound is preferably used because of theπ-electron excess heteroaromatic ring and the π-electron deficientheteroaromatic ring, for which the electron-transport property and thehole-transport property are high. Note that a material in which theπ-electron excess heteroaromatic ring is directly bonded to theπ-electron deficient heteroaromatic ring is particularly preferably usedbecause the donor property of the π-electron excess heteroaromatic ringand the acceptor property of the π-electron deficient heteroaromaticring are both increased and the energy difference between the S₁ leveland the T₁ level becomes small.

Further, when a plurality of light-emitting layers are provided andemission colors of the layers are made different, light emission of adesired color can be obtained from the light-emitting element as awhole. For example, the emission colors of first and secondlight-emitting layers are complementary in a light-emitting elementhaving the two light-emitting layers, so that the light-emitting elementcan be made to emit white light as a whole. Further, the same applies toa light-emitting element having three or more light-emitting layers.

Electron-Transport Layer 304

The electron-transport layer 304 contains a material with a highelectron-transport property.

The material with a high electron-transport property is preferably anorganic compound having a property of transporting more electrons thanholes, and is especially preferably a material with an electron mobilityof 10⁻⁶ cm²/V·s or more.

For example, it is possible to use a metal complex such astris(8-quinolinolato)aluminum(III) (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq), bis[2-(2-hydroxyphenyl)-benzoxazolato]zinc(abbreviation: Zn(BOX)₂), orbis[2-(2-hydroxyphenyl)-benzothiazolato]zinc (abbreviation: Zn(BTZ)₂).

Alternatively, it is possible to use a heteroaromatic compound such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: TAZ),3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: p-EtTAZ), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP), or4,4′-bis(5-inethylbenzoxazol-2-yl)stilbene (abbreviation: BzOs).

Further alternatively, it is possible to use a high molecular compoundsuch as poly(2,5-pyridinediyl) (abbreviation: PPy),poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation: PF-Py) orpoly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)](abbreviation: PF-BPy).

Electron-Injection Layer 305

The electron-injection layer 305 contains a material with a highelectron-injection property.

Examples of the material with a high electron-injection property includean alkali metal, an alkaline earth metal, a rare earth metal, and acompound thereof (e.g., an oxide thereof, a carbonate thereof, and ahalide thereof), such as lithium, cesium, calcium, lithium oxide,lithium carbonate, cesium carbonate, lithium fluoride, cesium fluoride,calcium fluoride, and erbium fluoride.

The electron-injection layer 305 may contain the above-describedmaterial with a high electron-transport property and a donor material.For example, the electron-injection layer 305 may be formed using an Alqlayer containing magnesium (Mg). When the material with a highelectron-transport property and the donor material are contained, themass ratio of the donor material to the material with a highelectron-transport property is from 0.001:1 to 0.1:1.

Examples of the donor material include an alkali metal, an alkalineearth metal, a rare earth metal, and a compound thereof (e.g., an oxidethereof), such as lithium, cesium, magnesium, calcium, erbium,ytterbium, lithium oxide, calcium oxide, barium oxide, and magnesiumoxide; a Lewis base; and an organic compound such as tetrathiafulvalene(abbreviation: TTF), tetrathianaphthacene (abbreviation: TTN),nickelocene, or decamethylnickelocene.

Charge-Generation Region

The charge-generation region included in the hole-injection layer andthe charge-generation region 308 each contains a material with a highhole-transport property and an acceptor material (electron acceptor).Note that the acceptor material is preferably added so that the massratio of the acceptor material to the material with a highhole-transport property is 0.1:1 to 4.0:1.

The charge-generation region is not limited to a structure in which amaterial with a high hole-transport property and an acceptor materialare contained in the same film, and may have a structure in which alayer containing a material with a high. hole-transport property and alayer containing an acceptor material are stacked. Note that in the caseof a stacked-layer structure in which the charge-generation region isprovided on the cathode side, the layer containing the material with ahigh hole-transport property is in contact with the cathode, and in thecase of a stacked-layer structure in which the charge-generation regionis provided on the anode side, the layer containing the acceptormaterial is in contact with the anode.

The material with a high hole-transport property is preferably anorganic compound having a property of transporting more holes thanelectrons, and is especially preferably an organic compound with a holemobility of 10⁻⁶ cm²/V·s or more.

Specifically, it is possible to use any of the materials with a highhole-transport property shown as materials that can be used for thehole-transport layer 302, such as aromatic amine compounds such as NPBand BPAFLP, carbazole derivatives such as CBP, CzPA, and PCzPA, aromatichydrocarbon compounds such as t-BuDNA, DNA, and DPAnth, and highmolecular compounds such as PVK and PVTPA.

Examples of the acceptor material include organic compounds, such as7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ) and chloranil, oxides of transition metals, and oxides ofmetals that belong to Groups 4 to 8 in the periodic table. Specifically,vanadium oxide, niobium oxide, tantalum oxide, chromium oxide,molybdenum oxide, tungsten oxide, manganese oxide, and rhenium oxide arepreferable since their electron-accepting property is high. Inparticular, use of molybdenum oxide is preferable because of itsstability in the atmosphere, a low hygroscopic property, and easilyhandling.

Electron-Injection Buffer Layer 306

The electron-injection buffer layer 306 contains a material with a highelectron-injection property. The electron-injection buffer layer 306facilitates electron injection from the charge-generation region 308into the EL layer 203. As the material having a high electron-injectionproperty, any of the above-described materials can be used.Alternatively, the electron-injection buffer layer 306 may contain anyof the above-described materials with a high electron-transport propertyand donor materials.

Electron-Relay Layer 307

The electron-relay layer 307 immediately accepts electrons drawn out ofthe acceptor material in the charge-generation region 308.

The electron-relay layer 307 contains a material with a highelectron-transport property. As the material with a highelectron-transport property, a phthalocyanine-based material or a metalcomplex having a metal-oxygen bond and an aromatic ligand is preferablyused.

As the phthalocyanine-based material, specifically, it is possible touse CuPc, a phthalocyanine tin(II) complex (SnPc), a phthalocyanine zinccomplex (ZnPc), cobalt(II) phthalocyanine, β-form (CoPc), phthalocyanineiron (FePc), or vanadyl 2,9,16,23-tetraphenoxy-29H,31H-phthalocyanine(PhO-VOPc).

As the metal complex having a metal-oxygen bond and an aromatic ligand,a metal complex having a metal-oxygen double bond is preferably used. Ametal-oxygen double bond has an acceptor property; thus, electrons cantransfer (be donated and accepted) more easily.

As the metal complex having a metal-oxygen bond and an aromatic ligand,a phthalocyanine-based material is also preferably used. In particular,vanadyl phthalocyanine (VOPc), a phthalocyanine tin(IV) oxide complex(SnOPc), or a phthalocyanine titanium oxide complex (TiOPc) ispreferable because a metal-oxygen double bond is more likely to act onanother molecule in terms of a molecular structure and an acceptorproperty is high.

As the phthalocyanine-based material, a phthalocyanine-based materialhaving a phenoxy group is preferably used. Specifically, aphthalocyanine derivative having a phenoxy group, such as PhO-VOPc, ispreferably used. The phthalocyanine derivative having a phenoxy group issoluble in a solvent; thus, the phthalocyanine derivative has anadvantage of being easily handled during formation of a light-emittingelement and an advantage of facilitating maintenance of an apparatusused for film formation.

Examples of other materials with a high electron-transport propertyinclude perylene derivatives such as 3,4,9,10-perylenetetracarboxylicdianhydride (abbreviation: PTCDA), 3,4,9,10-perylenetetracarboxylicbisbenzimidazole (abbreviation: PTCBI),N,N′-dioctyl-3,4,9,10-perylenetetracarboxylic diimide (abbreviation:PTCDI-C8H), N,N′-dihexyl-3,4,9,10-perylenetetracarboxylic diimide(abbreviation: Hex PTC), and the like. Alternatively, it is possible touse a nitrogen-containing condensed aromatic compound such aspirazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile (abbreviation:PPDN), 2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene(abbreviation: HAT(CN)₆), 2,3-diphenylpyrido[2,3-b]pyrazine(abbreviation: 2PYPR), or 2,3-bis(4-fluorophenyl)pyrido[2,3-b]pyrazine(abbreviation: F2PYPR). The nitrogen-containing condensed aromaticcompound is preferably used for the electron-relay layer 307 because ofits stability.

Further alternatively, it is possible to use7,7,8,8-tetracyanoquinodimethane (abbreviation: TCNQ),1,4,5,8-naphthalenetetracarboxylicdianhydride (abbreviation: NTCDA),pertluoropentacene, copper hexadecafluoro phthalocyanine (abbreviation:F₁₆CuPc),N,N′-bis(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl)-1,4,5,8-naphthalenetetracarboxylicdiimide (abbreviation: NTCDI-C8F),3′,4′-dibutyl-5,5″-bis(dicyanornethylene)-5,5″-dihydro-2,2′:5′,2″-terthiophene(abbreviation: DCMT), or a methanofullerene ([6,6]-phenyl C₆₁ butyricacid methyl ester).

The electron-relay layer 307 may further contain any of theabove-described donor materials. When the donor material is contained inthe electron-relay layer 307, electrons can transfer easily and thelight-emitting element can be driven at a lower voltage.

The LUMO levels of the material with a high electron-transport propertyand the donor material are preferably −5.0 eV to −3.0 eV, i.e., betweenthe LUMO level of the acceptor material contained in thecharge-generation region 308 and the LUMO level of the material with ahigh electron-transport property contained in the electron-transportlayer 304 (or the LUMO level of the EL layer 203 in contact with theelectron-relay layer 307 or with the electron-injection buffer layer306). When a donor material is contained in the electron-relay layer307, as the material with a high electron-transport property, a materialwith a LUMO level higher than the acceptor level of the acceptormaterial contained in the charge-generation region 308 can be used.

The above-described layers included in the EL layer 203 and theintermediate layer 207 can be formed separately by any of the followingmethods: an evaporation method (including a vacuum evaporation method),a transfer method, a printing method, an inkjet method, a coatingmethod, and the like.

By use of a light-emitting element described in this embodiment, apassive matrix light-emitting device or an active matrix light-emittingdevice in which driving of the light-emitting element is controlled by atransistor can be manufactured. Furtheunore, the light-emitting devicecan be applied to an electronic device, a lighting device, or the like.

Note that this embodiment can be combined with the other embodiment asappropriate.

This application is based on Japanese Patent Application serial no.2012-025834 filed with Japan Patent Office on Feb. 9, 2012, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. A light-emitting device comprising a light-emittinglayer, the light-emitting layer comprising: a phosphorescent compound;and a substance exhibiting thermally activated delayed fluorescence,wherein a fluorescence spectrum of the substance exhibiting thethermally activated delayed fluorescence overlaps with thelowest-energy-side absorption band in an absorption spectrum of thephosphorescent compound, and wherein an energy value of a peak of thefluorescence spectrum is lower than an energy value of a peak of thelowest-energy-side absorption band.
 3. The light-emitting deviceaccording to claim 2, wherein the phosphorescent compound is anorganometallic complex.
 4. The light-emitting device according to claim2, wherein the phosphorescent compound is an iridium complex.
 5. Anelectronic device comprising the light-emitting device according toclaim
 2. 6. A lighting device comprising the light-emitting deviceaccording to claim
 2. 7. A light-emitting device comprising alight-emitting layer, the light-emitting layer comprising: aphosphorescent compound; and a substance exhibiting thermally activateddelayed fluorescence, wherein a phosphorescence spectrum of thesubstance exhibiting the thermally activated delayed fluorescenceoverlaps with the lowest-energy-side absorption band in an absorptionspectrum of the phosphorescent compound, and wherein an energy value ofa peak of the fluorescence spectrum is lower than an energy value of apeak of the lowest-energy-side absorption band.
 8. The light-emittingdevice according to claim 7, wherein the phosphorescent compound is anorganometallic complex.
 9. The light-emitting device according to claim7, wherein the phosphorescent compound is an iridium complex.
 10. Anelectronic device comprising the light-emitting device according toclaim
 7. 11. A lighting device comprising the light-emitting deviceaccording to claim
 7. 12. A light-emitting device comprising alight-emitting layer, the light-emitting layer comprising: aphosphorescent compound; and a substance exhibiting thermally activateddelayed fluorescence, wherein a fluorescence spectrum and aphosphorescence spectrum of the substance exhibiting the thermallyactivated delayed fluorescence overlaps with the lowest-energy-sideabsorption band in an absorption spectrum of the phosphorescentcompound, and wherein an energy value of a peak of the fluorescencespectrum and an energy value of a peak of the phosphorescence spectrumare lower than an energy value of a peak of the lowest-energy-sideabsorption band.
 13. The light-emitting device according to claim 12,wherein the phosphorescent compound is an organometallic complex. 14.The light-emitting device according to claim 12, wherein thephosphorescent compound is an iridium complex.
 15. An electronic devicecomprising the light-emitting device according to claim
 12. 16. Alighting device comprising the light-emitting device according to claim12.